1. Field of the Invention
The present invention relates to an image pickup lens and particularly, to an image pickup lens comprising two lenses which can be reduced in size and weight while achieving a wider angle of view to be used for an image pickup device utilizing an image pickup element such as a CCD, a CMOS or the like to be mounted on a portable computer, a visual telephone, a cellular phone and the like.
2. Description of the Related Art
Recently, there is a remarkable development in the multimedia industry. For example, there has been an increasing demand for a camera utilizing an image pickup element such as a CCD, a CMOS or the like to be mounted on a portable computer, a visual telephone, a cellular phone and the like. Such camera needs to be mounted on a limited space. Thus, it is desirable that the camera be small in size and light in weight.
Accordingly, an image pickup lens used for such CCD camera is also required to be small and lightweight as well. Conventionally, the so-called one-lens system using a single lens is used as such image pickup lens.
The one-lens system can work sufficiently in the case where it is used for a solid image pickup element referred to as CIF with the resolution of approximately 110,000 pixels. Recently, however, a solid image pickup element referred to as VGA with a high resolution of approximately 300,000 pixels has been investigated for use. In order to sufficiently utilize the resolving power of the solid image pickup element with such high resolution, the conventional one-lens system is not sufficient.
Thus, conventionally, a variety of two-lens systems or three-lens systems, which are superior in regards to optical characteristic compared to the one-lens system, have been proposed.
By using the three-lens system, it is possible to effectively correct each aberration causing the deterioration of the optical characteristic. Therefore, an extremely high optical characteristic can be achieved. However, the three-lens system requires a large number of components. Thus, there raises problems that it is hard to reduce the size, and the manufacturing cost is increased since each component is required to be highly precise.
On the contrary, the two-lens system can achieve higher optical characteristic compared to the one-lens system even though it is not as high as that of the three-lens system. Thus, it can be considered as a small-sized and preferable lens system for a solid image pickup element with high resolution.
As the two-lens system, conventionally, a number of lens systems so-called retrofocus type in which a lens having a negative power and a lens having a positive power are combined have been proposed. With the retrofocus type lens system, it is possible to reduce the number of elements so that the cost can be decreased. However, it is practically impossible to reduce the size as small as that of the one-lens system due to such structure with the long back focus distance.
Further, as other two-lens system, there is a lens system so-called a telephoto type in which a lens having a positive power and a lens having a negative power are combined. However, the telephoto type lens system is originally developed for silver salt photographs so that the back focus distance is too short. Further, there is an issue of telecentricity. Thus, it is practically impossible to use it as the image pickup lens for the solid image pickup element.
Also, conventionally, a two-lens system in which two lenses having positive power are combined has been proposed. Examples of such optical systems are disclosed in, for example, Japanese Patent Application Laid-open No. 7-181379, No. 7-287164, No. 10-206725, and the like.
However, it is not possible to apply the conventional image pickup lens with two lenses in which two lenses having positive power are combined as it is for the solid image pickup element. The reason is that the lens systems disclosed in any of the above-mentioned Japanese Patent Application have been developed for silver salt photographs or optical systems such as a copying machine or fax machine. Therefore, the focal length is 20 mm or more, which is notably long. Also, it is an extremely dark lens system with F No. of 4.0 or more.